or maybe the effects are linked to the +. It protects from direct damage better with a higher + but also negates more and more of your own magic. So maybe its not an absolute negation like the CBFs skill/defense enchantment negation untill you get a huge upgraded one.

Shade, look at the CoBF description and then the mage shield description (gonna hafta think of something besides MS to call it :P). CoBF says it negates enchantments, while the shield says it negates spells.. Will the shield negate trained DD, or all spells regardless of type?